Organic electroluminescent devices, also known as organic light emitting diode (“OLED”) devices, comprise an anode, a cathode and an electroluminescent medium made up of extremely thin layers (typically less than 1.0 micrometer in combined thickness) separating the anode and the cathode. A basic two-layer light emitting diode comprises one organic layer that is specifically chosen to inject and transport holes and a second organic layer that is specifically chosen to inject and transport electrons. The interface between the two layers provides an efficient site for the recombination of the injected hole-electron pair, which results in electroluminescence. The electroluminescent medium can comprise additional layers, including, but not limited to, an emitter layer between the hole injection and transport and the electron injection and transport layers in which recombination of holes and electrons occurs. Since light emission is directly related to current density through the organic electroluminescent medium, the thin layers coupled with increased charge injection and transport efficiencies have allowed acceptable light emission levels (e.g., brightness levels capable of being visually detected in ambient light) to be achieved with low applied voltages in ranges compatible with integrated circuit drivers, such as field effect transistors.
One factor that may affect the commercial applications of OLEDs is the useful life span of the device. In particular, OLED luminance and life span are sometimes adversely impacted by efforts to improve device efficiency, such as, for example, by plasma treatment of the transparent or semitransparent conducting anode. Thus, gains in efficiency as a result of plasma treatment may result in reduced luminance and life span of the OLED. The reduction of initial luminance appears to relate to work function instabilities in the anode resulting from some efficiency-promoting processes. Accordingly, the luminance of many OLEDs (particularly those subjected to plasma treatment) degrades too rapidly with time to permit the OLEDs to be acceptable for commercial purposes. Therefore, there is a significant need for OLEDs with improved anode stability in terms of anode work function. OLEDs with improved anode stability are likely to produce luminance that falls off less rapidly with operation time, thereby increasing the useful life span of the OLEDs.